Top drives are used to suspend and rotate a string of drill pipe and/or casing in drilling applications. The top drive is supported by a drilling line wrapped on a set of sheaves and connected to drawworks at one extremity. The top drive supports the drill string via a thrust bearing. Mud may be pumped into the drill string via a swivel. Furthermore, the top drive generally includes one or more motors (electric or hydraulic) which generate(s) the rotation of the drill string. The reaction torque applied to the top drive may be transmitted to the mast via a set of rollers attached to the top-drive chassis.
Various measurements may be used to manage the drilling process, including those that involve the top drive. Hook load and hook elevation above the rig floor are two examples of such measurements. These measurements may be employed to calculate drilling parameters such as weight on bit (WOB), rate of penetration (ROP), and depth. A variety of other types of measurements are used to calculate these and other drilling parameters.
In many applications, sensors that take measurements such as these are distributed on the drilling rig, and the measurements taken may be indirect. Accordingly, non-linearities, reduced resolution, noise, etc. may be part of the measurement system and may or may not be corrected in the measurements. For example, hook load is generally measured on the “dead-line” of the drilling line, near or on the anchor below the rig floor. Generally, there is no correction for friction in the sheaves in this measurement. In addition, the weight of the travelling block, hook and top drive may limit the resolution of the hook-load measurement.
One way this is handled is by installing measurement and communication devices on top of the drill string. These devices thus rotate with the drill string. When such devices are installed, communication between these systems attached to (and rotating with) the drill string to the rig system may be performed either by rotary transformer (inductive coupling), or sliding contacts or e-mag communication (such as WIFI). The main node is thus at the rig, resulting in long communication paths form the rotating devices with risk of signal corruption.